A charge pump is an electronic circuit that uses switches to control the connections of voltages to double voltages, invert voltages, or generate arbitrary voltages, depending on the controller and circuit topology. An example for a charge pump circuit application is in phase locked loop circuits (PLL). For designing a charge pump circuit, an important objective is to minimize the mismatch between “up” (pull-up) and “down” (pull-down) currents. In an integer-N synthesizer, the current mismatch will cause the charge pump's output spectrum to have a higher reference spur. For fractional-N synthesizer, the current mismatch will cause an extra problem, known as higher in-band phase noise.
FIG. 1 shows an existing charge pump circuit that attempts to deal with the mismatch problem between currents “up” and “down”. In this circuit, the original current source I_bias is mirrored to a common current branch for presenting “up” (I_up) and “down” (I_down) currents to N-MOS transistor M1 and P-MOS transistor M2 respectively. Between these two current carrying transistors (M1, M2), there are four trans-gate switches (S1, S2, S3 and S4) of the same size and they form the current branch 101 (S1 and S2) parallel to a dummy branch 100 (S3 and S4). Each branch has its two trans-gate switches serially connected. The charge pump output voltage V_ds is taken at the CP_out point between S1 and S2 and a reference voltage V_ref is taken between S3 and S4. Linking charge pump output (V_ds) and the voltage reference V_ref, a negative feedback is formed via an Operational Amplifier (Op) so that the voltage value V_ref follows V_ds. In these branches, D and U are digital signals from a phase frequency detector (PFD) to control the trans-gate switches (S1 to S4) so that the pumping of the positive and negative current (CP_out) is regulated. In this circuit, a charge injection is minimized by implementing the identical switches (S1 to S4) with a minimal size and the possible overlap charge injection is reduced by fine-tuning the size of current carrying transistors (M1, M2). During operation, M1 and M2 are not switched on or off to prevent current switching effects on the drain of the current sources. When the charge pump is off i.e. both S1 and S2 are closed, the current is diverted into a dummy current branch 100 via S3 and S4.
In the charge pump circuit, there exists a systematic current variation due transistor mismatch between M1 and M2. Consequently, the resulting current mismatch of the charge pump circuit is in practice difficult to avoid.
Referring to FIG. 2, the simulation result for the charge pump circuit of FIG. 1 is shown. The vertical axis represents the electric current value and the horizontal axis gives the reference voltage V_ref (0˜1.8V) value, which follows the charge pump output voltage V_ds. The current passing through M1 (100 μA) is marked with I_up (curve 200) and the current passing through M2 (−95 μA) is marked with I_down (curve 202). The current mismatch is illustrated by the current_mismatch curve 204. It can be observed that the circuit is not able to compensate the current mismatch and resulted current mismatch is quite large.
Another existing charge pump circuit is illustrated in FIG. 3. The charge pump connects an original current source (I_bias) with a feedback network portion 300, a core charge pump portion 302 and a replica bias portion 304. This circuit uses the replica bias circuit 304 to equalize up and down currents regardless of the charge pump's output voltage V_ds. However, the voltage range V_ds of this charge pump is narrow which inhibits the feedback loop from operating properly. Such charge pump circuits cannot have good current match and are limited in terms of dynamic voltage range.
There are some charge pump circuits using digital circuits to control current mismatch. However, the digital circuit has to be turned on at all times to achieve good current match, which causes problems to the charge pump circuit.
A need therefore exists for compensating current mismatch in a charge pump circuit that seeks to address at least one of the above problems.